1. Field of the Invention
This invention relates to an electric device with a two-wire interface, which duplex-conductor interface serves the dual purpose of feeding electric power to the electric device and transmitting a signal, with the maximum power drawn by the electric device via said two-wire interface during normal operation being restricted to a predefined upper limit. The invention also relates to a method for operating an electric device of that type.
2. Background Information
Electric devices of the type referred to above, featuring a two-wire interface, are also called two-wire devices. Many of these two-wire devices are measuring instruments. In normal operation, as in a routine measuring process, these two-wire instruments usually draw a predefined amount of current. In two-wire devices with an analog power input port, that predefined current is typically in the range from 4 to 20 mA, where in the case of a measuring instrument 4 mA signals the lowest and 20 mA the highest value measured. In some cases, that range is expanded, with a span for instance from 3.8 to 20.5 mA, which assures the reliable detection of a measuring-range overrun for instance during calibration, or the transferability of the dynamics of a measuring signal. Readings outside that range are not interpreted as measured values. Instead, in many cases a value above the upper limit, 22 mA for example, is interpreted as an indication of a defect.
Two-wire devices whose two-wire interface is in the form of a digital bus connector, with the bus (for instance an IEC 61158-2) also serving to feed electric power to the two-wire device, usually draw current at a constant rate of about 10 mA. For two-wire interfaces in the form of an analog power connection as well as for two-wire interfaces in the form of a digital bus connector that feeds power to the two-wire device through the bus, the current for the electric device is provided by an outboard power supply.
Apart from routine operation of the electric device, for instance as a measuring instrument, the electric device must be able from time to time to perform certain special operational functions. Examples of such special operational functions may include software updates, i.e. the downloading, recording and/or programming of new software for the electric device. But other special operational functions may also include calibrating, diagnostic and maintenance processes.
Switching the electric device into any such special functional mode poses a problem insofar as electric devices whose power input connection is an analog two-wire interface cannot continue with their normal operation, such as a measuring process, while they are in the special function mode. Similarly, in electric devices with a two-wire interface in the form of a digital bus connector through which the electric device receives its power, there may be contention with normal operational functions, or at least the functionality of the electric device may be curtailed.